1. Field of the Invention
The present invention is directed to a method for evaluating a time signal that is generated as a magnetic resonance signal by means of magnetic resonance technology and that contains spectroscopic information.
2. Description of the Prior Art
Magnetic resonance spectroscopy has been utilized for more than four decades in basic physics, chemical and biochemical research, for example as an analysis technique or for structure clarification of complex molecules. Like magnetic resonance tomography, magnetic resonance spectroscopy is based on the principle of nuclear magnetic resonance. The primary objective of spectroscopy, however, is not imaging but the analysis of a substance. Resonant frequencies of isotopes that have a magnetic moment, for example 1H, 13C or 31P, are dependent on the chemical structure of molecules wherein the isotopes are bonded. A determination of the resonant frequencies therefore allows differentiation between various substances. The signal intensity at the various resonant frequencies provides information about the concentration of the corresponding molecules.
When a molecule is introduced into a basic magnetic field of a magnetic resonance apparatus, as occurs in spectroscopy, electrons of the molecules shield the basic magnetic field from atomic nuclei of the molecule. As a result of this effect, the local magnetic field at the location of an atomic nucleus changes by a few millionths of the external basic magnetic field. The variation of the resonant frequency of this atomic nucleus associated therewith is referred to as the chemical shift. Molecules thus can be identified on the basis of their chemical shift. Since frequency differences can be acquired by measurement more simply and more precisely than absolute frequencies, the chemical shift is identified in ppm relative to a reference signal, for example the operating frequency of the magnetic resonance apparatus.
A resonance line of an atomic nucleus can be split into a number of lines when further atomic nuclei with a magnetic moment are situated in the environment of the atomic nucleus under observation. The cause of this is the spin-spin coupling between the atomic nuclei. The magnetic flux density of the basic magnetic field that an atomic nucleus experiences thus is not only dependent on the electron envelope around this atomic nucleus but also is dependent on the orientation of the magnetic fields of the neighboring atoms.
Clinical magnetic resonance spectroscopy is magnetic resonance spectroscopy using a clinical magnetic resonance apparatus. The methods of localized magnetic resonance spectroscopy basically differ from those of magnetic resonance imaging by the chemical shift also being resolved in spectroscopy in addition to the tomographic spatial resolution. Two localization methods of magnetic resonance spectroscopy currently dominate in the clinical application. A first of these, discrete (or individual or single) volume techniques based on echo methods wherein a spectrum of a previously selected target volume is recorded. As second of these are spectroscopic imaging methods, referred to as CSI methods (chemical shift imaging), which simultaneously enable the recording of spectra of many spatially interconnected target volumes.
Spectroscopic examination methods are employed in clinical phosphorous spectroscopy as well as in proton spectroscopy. For example, a three-dimensional CSI method includes the following steps: After a non-slice-selective 90° RF pulse, a combination of magnetic phase-encoding gradients of the three spatial directions is activated for a defined time, and the magnetic resonance signal is subsequently read out in the absence of any and all gradients. This is repeated with other combinations of phase-encoding gradients until the desired spatial resolution has been achieved. A four-dimensional Fourier transformation of the magnetic resonance signals supplies the desired spatial distribution of the resonance lines. A two-dimensional CSI method arises from the above-described three-dimensional method by the aforementioned, non-slice-selective RF pulse being replaced by a slice-selective excitation composed of a slice-selective RF pulse and a corresponding magnetic gradient, and the phase-encoding direction is eliminated.
The discrete volume techniques that are usually employed are based on the acquisition of a stimulated echo or of a secondary spin echo. In both instances, a spatial resolution ensues by means of successive selective excitations of three orthogonal slices. A target volume is defined by a section volume of the three slices. Only the magnetization of the target volume experiences all three selective RF pulses and thus contributes to the stimulated echo or secondary spin echo. The spectrum of the target volume is obtained by one-dimensional Fourier transformation of a time signal corresponding to the stimulated echo or to the secondary spin echo.
The intense water signals are often suppressed in clinical proton spectroscopy. For example, one method of such water suppression is the CHESS technique, wherein the nuclear spins of the water molecules are first selectively excited by narrow-band 90° RF pulses and their transverse magnetization is subsequently dephased by the activation of magnetic field gradients. A detectable magnetization of the water molecules is thus no longer available—in the ideal case—for a spectroscopy method that follows immediately thereafter.
For a prescribable volume to be examined, for example, a magnetic resonance signal is generated with one of the methods described above, this magnetic resonance signal being acquired in the time domain and being converted into an appertaining spectrum by a Fourier transformation, with, for example, a real part or (the magnitude) of the spectrum being presented. The spectrum is characterized by resonance lines that are referred to as spikes. These resonance lines or spikes usually occur in the form of narrow, bell-shaped curves. Each of the resonance lines or spikes can have a maximum amplitude value allocated to it that in turn defines an appertaining frequency value of the resonance line that is characteristic of the resonance line, and thus of a very specific magnetic resonance signal-emitting substance contained in the volume. Further, an integral value for one of the resonance lines or spikes in an absorption spectrum provides information about the concentration that the appertaining substance has in the volume under examination.
It is unavoidable in the practical application of spectroscopic methods that noise signals also are acquired as a consequence of the method and also contained in the magnetic resonance signal in addition to informational signals of interest. The amplitude of the noise signals can even exceed the informational signals by a multiple, which also degrades the interpretation of the informational signals when their characteristic frequencies clearly differ from characteristic frequencies of the noise signals because the resonance lines of the informational signals can experience an overlap in the region of a broad base region of an excessively powerful resonance line of a noise signal component. Further, a quantification of a base line is made more difficult as a result.
One group of methods for eliminating noise signals is based on a technique known as parameterization or a technique referred to as a fitting and subsequent subtraction of the noise signals. More recent methods of this type operate in the time domain. A common feature of these methods is the prescription of a model function of the noise signals is prescribed. Such methods are known, for example, from the article by W. W. F. Pijnappel et al., “SVD-Based Quantification of Magnetic resonance Signals”, Journal of Magnetic resonance 97 (1992), pages 122-134, and from the article by E. Cabanes et al., “Optimization of residual Water Signal Removal by HLSVD on Simulated Short Echo Time Proton MR Spectra of the Human Brain”, Journal of Magnetic Resonance 150 (2001), pages 116-125. These methods are based on the assumption that the noise signals can be presented as a sum of exponentially attenuated signals (Lorentz lines).
In an apparatus operating according to these methods, for example, a computational water suppression is implemented during the course of post-processing by a signal part derived from water being modeled as a noise signal by means of a polynomial function (spline function) in the time domain and being subsequently subtracted. These methods are not free of pre-conditions with respect to the line shape of the noise signals. Different noise signals meet these preconditions to different extents. The methods usually function well for narrow-band water signals but markedly less well for broadband fat signals.
Methods referred to as filter methods are known as a further group for eliminating noise signals. Such methods are disclosed, for example, in the article by T. Sundin et al., “Accurate Quantification of 1H Spectra: From Finite Impulse Response Filter Design for Solvent Suppression to Parameter Estimation”, Journal of Magnetic Resonance 139 (1999), pages 189-204, and in the article by A. Coron et al., “The Filtering Approach to Solvent Peak Suppression in MRS: A Critical Review”, Journal of Magnetic resonance 152 (2001), pages 26-40. In this group of methods, as well, it is assumed in the design of the filters that the informational signals and the noise signals are exponentially attenuated signals.
German OS 100 12 278 discloses a method for the operation of a magnetic resonance apparatus wherein a magnetic resonance signal is acquired for a time span, and whereby the magnetic resonance signal in the time domain is subjected to a Fourier transformation for generating a magnetic resonance spectrum. The magnetic resonance signal is weighted before the Fourier transformation with a bell-shaped window function that has a window width that is smaller than or equal to the time span and that is placed into a time range of the time span. In one embodiment, the bell-shaped window function is a symmetrical window function, for example a Hanning window function. Spectroscopy methods usually generate non-time-symmetrical time signals, i.e. asymmetrical time signals, which cause a spread of resonance lines in the spectrum. As a result of the method disclosed in German OS 100 12 278, the magnetic resonance signal is made symmetrical and a spread of the resonance lines is thereby prevented. The usefulness of spectra generated in this way, however, is limited because, among other things, the informational signals exhibit lower signal amplitudes compared to the non-symmetrical spectrum.